In order to ascertain that the thymus is the organ in which the T cell self-MHC specific repertoire is determined, the MHC specificity of cytotoxic precursor cells differentiating in the thymuses of radiation bone marrow chimeras was assessed. The results of the experiments demonstrated that thymocytes from A yields AxB chimeras were capable of recognizing TNP in association with both A and B MHC determinants, and that thymocyte from AxB yields A chimeras only recognized TNP in association with A, not B, MHC determinants. Thus, the earliest donor T cell in chimeric mice already displayed self-MHC recognition restricted to the host thymic MHC determinants. These results are consistent with the thymus being the site where MHC restrictions of T cell recognition are determined, and virtually excluded the possibility that MHC restriction is only a post-thymic event.